The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well-known and is, in fact, the method used currently for commercial production of ethylene oxide. The typical catalyst for such purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain a porous, inert support or carrier such as alpha alumina upon which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. in Catalyst Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
It is also well-known, however, that the catalysts and reaction conditions which are best suited for ethylene oxide production do not give comparable results in the direct oxidation of higher olefins such as propylene. The discovery of processes capable of providing propylene oxide by vapor phase direct oxidation in higher yields than are presently attainable thus would be most desirable.
The use of supported silver catalysts containing relatively low levels of alkali metal promoters and rhenium promoters for direct oxidation of ethylene to ethylene oxide has previously been proposed. For example, U.S. Pat. No. 4,833,261 teaches supported silver catalysts containing a maximum of 3000 ppm alkali metal and 1860 ppm rhenium. The support, according to the patent, is preferably of aluminous composition; the use of an alkaline earth metal carbonate or titanate as a support is not suggested. The alkali metal promoter may be introduced using any of a large number of alkali metal compounds; the choice of counter anion to the alkali metal was apparently not regarded as critical. The patent also discloses that diluents such as carbon dioxide and moderating agents such as dichloroethane may be introduced in the feedstream contacted with the catalyst. The use of a nitrogen oxide species such as NO is not taught, however. Moreover, there is no suggestion in the patent that these catalysts or reaction conditions could successfully be modified or adapted in order to selectively convert propylene to propylene oxide.